More than 100,000 asteroids hurtle past our planet. But only one—that we know of—may hit us in the next 30 years.

Friday the 13th of April 2029 could be a very unlucky day for planet Earth. At 4:36 am Greenwich Mean Time, a 25-million-ton, 820-ft.-wide asteroid called 99942 Apophis will slice across the orbit of the moon and barrel toward Earth at more than 28,000 mph. The huge pockmarked rock, two-thirds the size of Devils Tower in Wyoming, will pack the energy of 65,000 Hiroshima bombs--enough to wipe out a small country or kick up an 800-ft. tsunami.

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On this day, however, Apophis is not expected to live up to its namesake, the ancient Egyptian god of darkness and destruction. Scientists are 99.7 percent certain it will pass at a distance of 18,800 to 20,800 miles. In astronomical terms, 20,000 miles is a mere stone's throw, shorter than a round-trip flight from New York to Melbourne, Australia, and well inside the orbits of Earth's many geosynchronous communications satellites. For a couple of hours after dusk, people in Europe, Africa and western Asia will see what looks like a medium-bright star creeping westward through the constellation of Cancer, making Apophis the first asteroid in human history to be clearly visible to the naked eye. And then it will be gone, having vanished into the dark vastness of space. We will have dodged a cosmic bullet.

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Maybe. Scientists calculate that if Apophis passes at a distance of exactly 18,893 miles, it will go through a "gravitational keyhole." This small region in space--only about a half mile wide, or twice the diameter of the asteroid itself--is where Earth's gravity would perturb Apophis in just the wrong way, causing it to enter an orbit seven-sixths as long as Earth's. In other words, the planet will be squarely in the crosshairs for a potentially catastrophic asteroid impact precisely seven years later, on April 13, 2036.

Radar and optical tracking during Apophis's fly-by last summer put the odds of the asteroid passing through the keyhole at about 45,000-to-1. "People have a hard time reasoning with low-probability/high-consequence risks," says Michael DeKay of the Center for Risk Perception and Communication at Carnegie Mellon University. "Some people say, 'Why bother, it's not really going to happen.' But others say that when the potential consequences are so serious, even a tiny risk is unacceptable."

Former astronaut Rusty Schweickart, now 71, knows a thing or two about objects flying through space, having been one himself during a spacewalk on the Apollo 9 mission in 1969. Through the B612 Foundation, which he co-founded in 2001, Schweickart has been prodding NASA to do something about Apophis--and soon. "We need to act," he says. "If we blow this, it'll be criminal."

If the dice do land the wrong way in 2029, Apophis would have to be deflected by some 5000 miles to miss the Earth in 2036. Hollywood notwithstanding, that's a feat far beyond any current human technology. The fanciful mission in the 1998 movie Armageddon--to drill a hole more than 800 ft. into an asteroid and detonate a nuclear bomb inside it--is about as technically feasible as time travel. In reality, after April 13, 2029, there would be little we could do but plot the precise impact point and start evacuating people.

Diagram: How to Off An Asteroid

Fortunately, Apophis needs to be nudged only about a mile to avoid a gravitational "keyhole" in space--a region that would send the asteroid on a collision course with Earth. Otherwise, it would have to be diverted 5000 miles for it to miss our planet. This reduces the energy required to deflect Apophis by a factor of about 10,000--making it theoretically possible using current technology. A number of methods have been proposed to do the job.

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According to projections, an Apophis impact would occur somewhere along a curving 30-mile-wide swath stretching across Russia, the Pacific Ocean, Central America and on into the Atlantic. Managua, Nicaragua; San José, Costa Rica; and Caracas, Venezuela, all would be in line for near-direct hits and complete destruction. The most likely target, though, is several thousand miles off the West Coast, where Apophis would create a 5-mile-wide, 9000-ft.-deep "crater" in the water. The collapse of that transient water crater would trigger tsunamis that would hammer California with an hour-long fusillade of 50-ft. waves.

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BUT DON'T EVACUATE just yet. Although we can't force Apophis to miss the Earth after 2029, we have the technology to nudge it slightly off course well before then, causing it to miss the keyhole in the first place. According to NASA, a simple 1-ton "kinetic energy impactor" spacecraft thumping into Apophis at 5000 mph would do the trick. We already have a template for such a mission: NASA's Deep Impact space probe--named after another 1998 cosmic-collision movie--slammed into the comet Tempel 1 in 2005 to gather data about the composition of its surface. Alternatively, an ion-drive-powered "gravity tractor" spacecraft could hover above Apophis and use its own tiny gravity to gently pull the asteroid off course.

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In 2005, Schweickart urged NASA administrator Michael Griffin to start planning a mission to land a radio transponder on Apophis. Tracking data from the device would almost certainly confirm that the asteroid won't hit the keyhole in 2029, allowing everyone on Earth to breathe a collective sigh of relief. But if it didn't, there still would be time to design and launch a deflection mission, a project that Schweickart estimates could take as long as 12 years. It would need to be completed by about 2026 to allow enough time for a spacecraft's tiny nudge to take effect.

NASA, however, is taking a wait-and-see attitude. An analysis by Steven Chesley of the Near Earth Object program at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., concludes that we can safely sit tight until 2013. That's when Apophis swings by Earth in prime position for tracking by the 1000-ft.-dia. radio telescope in Arecibo, Puerto Rico. This data could also rule out a keyhole hit in 2029. But if it doesn't, the transponder mission and, if necessary, a last-resort deflection mission could still be launched in time, according to Chesley. "There's no rush right now," he says. "But if it's still serious by 2014, we need to start designing real missions."

About 100 tons of interplanetary material drifts to the Earth's surface on a daily basis. Occasionally, an object hurtles with enough force to leave a mark.

ASTEROIDS are large rocky or metal bodies that originate in the relatively warm inner solar system, in the region between the orbits of Mars and Jupiter.

COMETS are composed mostly of water ice and rock, and form in the cold outer solar system beyond the planets' orbits. Scientists believe comets may have delivered the first organic compounds to Earth billions of years ago.

METEOROIDS are either pieces of asteroids that collided in space, or debris released by vaporizing comets. When meteoroids enter Earth's atmosphere, they are called meteors, and when they reach its surface they are called meteorites. So far, the remnants of more than 160 impact craters have been identified on Earth. Here are six of the most notable:

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50 THOUSAND YEARS AGO

35 MILLION YEARS AGO

35.7 MILLION YEARS AGO

65 MILLION YEARS AGO

1.85 BILLION YEARS AGO

2 BILLION YEARS AGO

VREDEFORT DOME

South Africa

Diameter: 236 miles

Cause: 6-mile-wide comet

Claim to fame: Though now the most eroded, Vredefort is the oldest and (at impact) the largest such crater on Earth. It was created by the world's greatest known energy release, which may have altered the evolution of single-cell organisms.

IN 1998, CONGRESS mandated NASA to find and track near-Earth asteroids at least 1 kilometer in diameter. The resulting Spaceguard Survey has detected, at last count, about 75 percent of the 1100 estimated to be out there. (Although Apophis was nearly 2500 ft. short of the size criterion, it was found serendipitously during the search process.) Thankfully, none of the giants so far discovered is a threat to Earth. "But any one of those couple of hundred we haven't found yet could be headed toward us right now," says former astronaut Tom Jones, an asteroid-search consultant for NASA and a Popular Mechanics editorial adviser. The space agency plans to expand Spaceguard to include asteroids down to 140 meters in diameter—less than half the size of Apophis, but still big enough to do serious damage. It has already detected more than 4000 of these; NASA estimates approximately 100,000 exist.

Predicting asteroid orbits can be a messy business, as the history of tracking Apophis in its 323-day orbit demonstrates. Astronomers at Arizona's Kitt Peak National Observatory discovered the asteroid in June 2004. It was six months before additional sightings—many made by amateurs using backyard telescopes—triggered alarm bells at JPL, home to the Sentry asteroid-impact monitoring system, a computer that predicts the orbits of near-Earth asteroids based on astronomical observations. Sentry's impact predictions then grew more ominous by the day. On Dec. 27, 2004, the odds of a 2029 impact reached 2.7 percent—a figure that stirred great excitement in the small world of asteroid chasers. Apophis vaulted to an unprecedented rating of 4 on the Torino Impact Hazard Scale, a 10-step, color-coded index of asteroid and comet threat levels.

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But the commotion was short-lived. When previously overlooked observations were fed into the computer, it spit out reassuring news: Apophis would not hit the Earth in 2029 after all, though it wouldn't miss by much. Oh, and there was one other thing: that troublesome keyhole.

The small size of the gravitational keyhole—just 2000 ft. in diameter—is both a blessing and a curse. On the one hand, it wouldn't take much to nudge Apophis outside it. Calculations suggest that if we change Apophis's velocity by a mere 0.0001 mph—about 31 in. per day—in three years its orbit would be deflected by more than a mile, a piddling amount, but enough to miss the keyhole. That's easily within the capabilities of a gravity tractor or kinetic energy impactor. On the other hand, with a target so minuscule, predicting precisely where Apophis will pass in relation to the keyhole becomes, well, a hit-or-miss proposition. Current orbit projections for 2029 have a margin of error—orbital scientists call it the error ellipse—of about 2000 miles. As data rolls in, the error ellipse will shrink considerably. But if the keyhole stubbornly stays within it, NASA may have to reduce the ellipse to a mile or less before it knows for sure whether Apophis will hit the bull's-eye. Otherwise, a mission risks inadvertently nudging Apophis into the keyhole instead of away from it.

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Can we predict Apophis's orbit to the submile level far enough in advance to launch a deflection mission? That level of forecasting accuracy would require, in addition to a transponder, a vastly more complex orbital calculation model than the one used today. It would have to include calculations for such minute effects as solar radiation, relativity and the gravitational pulls of small nearby asteroids, none of which are fully accounted for in the current model.

And then there's the wild card of asteroid orbital calculations: the Yarkovsky Effect. This small but steady force occurs when an asteroid radiates more heat from one side than the other. As an asteroid rotates away from the sun, the heat that has accumulated on its surface is shed into space, giving it a slight push in the other direction. An asteroid called 6489 Golevka, twice the size of Apophis, has been pushed about 10 miles off course by this effect in the past 15 years. How Apophis will be influenced over the next 23 years is anybody's guess. At the moment we have no clue about its spin direction or axis, or even its shape—all necessary parameters for estimating the effect.

IF APOPHIS IS INDEED headed for the gravitational keyhole, ground observations won't be able to confirm it until at least 2021. By that time, it may be too late to do anything about it. Considering what's at stake—Chesley estimates that an Apophis-size asteroid impact would cost $400 billion in infrastructure damage alone—it seems prudent to start taking steps to deal with Apophis long before we know whether those steps will eventually prove necessary. When do we start? Or, alternatively, at what point do we just cross our fingers and hope it misses? When the odds are 10-to-1 against it? A thousand-to-1? A million?

When NASA does discover a potentially threatening asteroid like Apophis, it has no mandate to decide whether, when or how to take action. "We're not in the mitigation business," Chesley says. A workshop to discuss general asteroid-defense options last June was NASA's first official baby step in that direction.

If NASA eventually does get the nod—and more important, the budget—from Congress, the obvious first move would be a reconnaissance mission to Apophis. Schweickart estimates that "even gold-plated at JPL," a transponder-equipped gravity tractor could be launched for $250 million. Ironically, that's almost precisely the cost of making the cosmic-collision movies Armageddon and Deep Impact. If Hollywood can pony up a quarter of a billion in the name of defending our planet, why can't Congress?